The ring-opening polymerization of tetrahydrofuran was reported in 1937 by Meerwein, using a trialkyloxonium salt as a catalyst. A review by Meerwein of the polymerization of tetrahydrofuran is found in Angew. Chem. 72, 927 (1960).
In subsequent years a number of other researchers have contributed to knowledge of tetrahydrofuran polymerization catalysts and mechanisms.
Generally, initiation requires the formation, in some manner, of a THF oxonium ion. The species R ##STR1## must be formed in order for the propagation reaction to take place. The different ways in which this species can be formed are conveniently grouped as follows:
1. Reaction with preformed trialkyl oxonium salts.
2. In situ formation of oxonium ion.
3. Addition of a carbonium ion.
4. Hydrogen abstraction (Adv. Poly. Sci., 4, 528 (1967). page 537.)
Ibid at page 531, there is a discussion of catalysts for tetrahydrofuran polymerization. These include combinations of metal halides, unsaturated tertiary oxonium salts, complex inorganic acids, catalysts of Lewis acid class, such a PF.sub.5, alkyl ammonium compounds used with water or epichlorohydrin, and certain related complex ions. At page 536 there is a discussion of the "Mechanism of Polymerization."
In an article by M. P. Dreyfuss et al., titled "The Reaction of Trityl Salts with 2-Methyltetrahydrofuran and with Tetrahydrofuran", in Macromolecules, 1, 437 (1968), there is a discussion of the use of trityl salts as initiators in the preparation of cyclic ethers. Trityl salts include, for example, Ph.sub.3 CClO.sub.4 and Ph.sub.3 CPF.sub.6. One disadvantage of using these salts to polymerize tetrahydrofuran, as noted at page 440, is that the products are characteristically dark colored.
Oxonium ions have been studied as initiators. These ions are discussed in an article titled "The Mechanism of the Initiation Reaction of the Cationic Polymerization of Tetrahydrofuran by Stable Cationic Salts", by Y. Yamashita, et al., in Die Makromolekulare Chemie, 142, 171-181 (1971). At page 172 it is noted that triethyloxonium salts were shown to initiate the polymerization of tetrahydrofuran by the straight forward alkyl exchange reaction. In the examples of this reference it is noted on page 175 that the reactivity of the cyclic ethers could be correlated with basicity.
Metal salts as initiators are discussed in "Use of Mono-and Multifunctional Oxocarbenium Salts in the Polymerization of Tetrahydrofuran", by E. Franta, et al., J. Polymer Sci.: Symposium No. 56, 139-148 (1976). In this work oxocarbonium salts were found useful as initiators for the polymerization of tetrahydrofuran and allowed the polymerization to proceed without termination. Mono- and difunctional initiators were prepared which reacted rapidly and were used to prepare block copolymers and graft copolymers of controlled structure.
In a book titled "Polymerization of Aldehydes and oxides", J. Furukawa et al., Interscience Publishers, 1963, Chapter V is a review titled "Polymerizations of Tetrahydrofuran and 1,4-Epoxycyclohexane." It is noted at page 225 that the polymer of tetrahydrofuran is a mobile liquid and a viscous liquid, or a solid, depending on its molecular weight. The softening point of the solid polymer is so low (41.degree..+-.20.degree. C.) that it has no practical applications without any processing. This reference provides a review of the various classifications of trialkyloxonium salt catalysts used in the polymerization of tetrahydrofuran. A model is projected for the polymerization of a substituted tetrahydrofuran wherein the melting points and other properties could be manipulated.
U.S. Pat. No. 2,856,370, to Muetterties, discloses a polytetrahydrofuran prepared with a phosphorous pentafluoride catalyst, having a melting point above 100.degree. C.
E. P. 428,003 A2 discloses the use of heteropolyacid catalysts in polymerization of tetrahydrofuran.
P. Dreyfuss has authored a reference titled "Poly(tetrahydrofuran)", Gordon and Breach Science Publishers, 1982. Chapter seven contains a discussion of the industrial applications of polytetrahydrofuran in polyurethane and polyester thermoplastic elastomers. At page 194 it is stated that commercial polyurethanes and polyesters containing polytetrahydrofuran are usually based on .alpha.,.omega.-difunctional precursors of relatively low molecular weight which are most often diprimary diols. An example is poly(tetramethylene ether) glycol(PTMEG).
At page 196, Ibid, there is a discussion of methods of initiating polymerization of tetrahydrofuran with fluorosulfonic acid to yield PTHF in which it is possible to have hydroxyl groups at both ends after treatment with water. A noted disadvantage is that acids used for initiation cannot be recovered for further use and disposal of the acidic by-products is a problem, due to their toxicity and corrosiveness. Strong acids can cause the degradation of the PTMEG. In addition, the presence of hydroxyl groups during polymerization markedly increases molecular weight and broadens the molecular weight distribution. Some of these disadvantages can be overcome by polymerization of tetrahydrofuran using a strong protonic acid in the presence of acetic anhydride. It is mentioned at page 197 that Nafion resin (NfSO.sub.3 H) may be used as a strong-acid catalyst and that when PTHF glycols are prepared using this resin, the reaction mixture normally contains acetic anhydride and acetic acid in addition to tetrahydrofuran. Data indicates that the ratio of anhydride to acid serves as a means of controlling the molecular weight of resulting PTHF diacetates. At page 200, there is disclosed the use of an anhydride of a very strong acid as an initiator to form PTHF having oxonium ions on both ends of the chain which can be treated with water to form PTMEG. The use of Polymer Polyols for High Performance urethane elastomers is discussed at page 197. No. 10131/8.
The use of clays as catalysts for selected applications is known in the art. In an article titled "Catalysis: Selective Developments", Chem. Systems Report 84-3, 239-249, at section 3.4320, the unusual properties of smectite clays which make them of interest as catalysts are discussed. These compositions are layered and exhibit a 2:1 relationship between tetrahedral and octahedral sites. In addition the combination of cation exchange, intercalation and the fact that the distance between the layers can be adjusted provide interesting possibilities.
There is a discussion of clay mineral catalysts, including "acid" montmorillonite clay catalysts in "Progress in Inorganic Chemistry", Vol. 35, p. 41 (1987). The process of pillaring this type of catalyst is discussed. Pillaring can convert a clay lamellar solid into a more heat resistant two dimensional zeolite material.
G. B. Patent No. 2,179,563 (1987) discloses the use of modified layered clay catalysts in reactions capable of catalysis by protons. Of particular interest in this invention were the three-layer sheet types, such as smectites, micas and vermiculites composed of successive layers of tetrahedral silica, octahedral alumina and tetrahedral silica which can exhibit swelling properties.
U.S. Pat. No. 4,590,294 discloses a process for the production of an ester comprising reacting an olefin from the group consisting of ethylene, hex-1-ene, hept-1-ene, oct-1-ene, 4-methylpent-1-ene, hex-2-ene, 1,5-hexadiene and cyclohexene with a carboxylic acid using as a catalyst component a hydrogen ion-exchanged layered clay.
Copending U.S. patent application Ser. No. 07/494,280 discloses the reaction of butanol and methanol in the presence of acidic montmorillonite clay catalysts having certain identifiable physical parameters, such as surface area, acidity range and moisture content.
In U.S. Pat. No. 4,189,566 there is disclosed a process for the preparation of polybutylene glycol carboxylic acid esters by polymerizing tetrahydrofuran, wherein the tetrahydrofuran, after removal of the catalyst used for preparation of tetrahydrofuran, is treated before polymerization with a strong mineral acid, an organic sulfonic acid, silica gel and/or a bleaching earth and is then polymerized in the presence of one or more carboxylic acids and/or carboxylic acid anhydrides and a polymerization catalyst.
U.S. Pat. No. 4,243,799 discloses an improvement of U.S. Pat. No. 4,189,566 wherein a bleaching earth is used which contains less than 3% by weight of water, said catalyst being arranged in a fixed bed and passing a mixture of pretreated tetrahydrofuran and carboxylic anhydride through said fixed bed.
The object of this improvement is to decrease the amount of discoloration and to affect the selectivity for certain molecular weights.
None of the cited references suggest the use of montmorillonite clay catalysts in the polymerization of cyclic ethers, and especially not tetrahydrofuran.
From the references reviewed above it would appear that methods of polymerizing tetrahydrofuran are of great interest in the art. It is also noted from the description of relevant research that there have been attempts to produce polymerization products having greater viscosity and to find catalysts which enhance conversion, yet exhibit fewer disadvantages. In addition, improvements in rate of conversion are always desirable commercially.
It would be a distinct advance in the art if a catalyst were available which allowed for an improved rate of polymerization of tetrahydrofuran, improved viscosity and improved conversion while producing fewer acid by-products with their inherent disadvantages. These properties would also make the products more useful in injection molding techniques, see "Poly(tetrahydrofuran)", supra, p. 238.
It has now been discovered that the use of a triflic acid-modified montmorillonite clay in the polymerization of tetrahydrofuran results in an improved rate of conversion and in increased viscosity of the liquid polyether glycol products.